The present invention relates to a semiconductor light-emitting device applicable to, for example, a short-wavelength light-emitting diode and a method for fabricating-the device.
In the case of GaN, for example, gallium nitride (GaN)-based Group III-V nitride semiconductor has a large band gap of 3.4 eV at room temperature and, therefore, is applicable to light-emitting devices such as visible-range light-emitting diodes and short-wavelength semiconductor laser diodes. Especially for light-emitting diodes, white-light-emitting diodes that emit light by exciting fluorescent materials with blue-green or blue emitted light have been put in practical use for various displays or display apparatus.
GaN-based light-emitting diodes, which are expected to expand their market in future, conventionally use epitaxial layers grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) to date. As a result of establishment of elemental technology such as: heteroepitaxial growth in which a low-temperature buffer layer is provided on a sapphire substrate; growth of an active layer having a multi-quantum-well structure made of indium gallium nitride (InGaN); and growth of a low-resistance p-type GaN layer by activation annealing, characteristics of the GaN-based light-emitting diodes have been improved.
However, sapphire substrates have various drawbacks such as the absence of conductivity and low heat dissipation, so that reduction of an operating voltage and enhancement of power are limited. To eliminate such various drawbacks of GaN-based light-emitting devices using sapphire substrates, a so-called laser lift-off technique is developed. With this technique, high-power short-wavelength pulse laser light is applied to the surface of a sapphire substrate opposite a GaN-based epitaxial layer, for example, so that a GaN layer near the interface between the substrate and the epitaxial layer is thermally decomposed, thereby separating the epitaxial layer. This enables a light-emitting diode having a vertical structure in which a sapphire substrate is separated to be implemented.
With this structure, if a metal supporting substrate made of, for example, a relatively-thick gold plating material or copper tungsten (CuW) and exhibiting excellent heat dissipation is attached to an epitaxial layer, heat is dissipated through the metal supporting substrate so that power is easily increased. In addition, a vertical electrode structure on which electrodes are formed on both faces of the epitaxial layer is allowed to be adopted, a series resistance and an operating voltage are advantageously reduced. It is expected that employment of a so-called free standing structure in which a substrate for crystal growth is separated is indispensable for enhancement of performance of GaN-based light-emitting diodes in future.
However, the conventional nitride semiconductor light-emitting device with the free standing structure has the following drawbacks. For example, in a light-emitting diode disclosed in T. Ueda et al., Phys. Stat. Sol. 0, No. 7 pp. 2219-2222 (2003), a diode chip is held only by a gold plating material, so that the strength of the diode chip becomes insufficient to cause a crack or other defects of the chip during packaging. In addition, a high-reflectance electrode is provided on a p-type semiconductor layer, so that the light extracting efficiency is enhanced. However, means for extracting light laterally leaked in the chip is not provided, so that increase of luminance is limited.
Further, in a GaN-based light-emitting diode disclosed in, for example, D. Morita et al., Jpn. J. Appl. Phys. 41 pp. L1434-1436 (2002), an epitaxial layer is attached to a supporting substrate made of CuW and then a sapphire substrate for crystal growth is separated. That is, the process step of attaching the supporting substrate to the epitaxial layer is added. Accordingly, cost reduction of the chip is limited.